Aging of the human skeleton is characterized by decreased boneformation and bone mass and these changes are more pronounced inpatients with osteoporosis. As osteoblasts and adipocytes share a commonprecursor cell in the bone marrow, we hypothesized that decreased boneformation observed during aging and in patients with osteoporosis is theresult of enhanced adipognesis versus osteoblastogenesis from precursorcells in the bone marrow. Thus, we examined iliac crest bone biopsiesobtained from 53 healthy normal individuals (age 30–100) and 26patients with osteoporosis (age 52–92). Adipose tissue volumefraction (AV), hematopoietic tissue volume fraction (HV) and trabecularbone volume fraction (BV) were quantitated as a percentage of totaltissue volume fraction (TV) (calculated as BV + AV + HV) usingthe point-counting method. We found an age-related increase in AV/TV(r = 0.53, p < 0.001, n =53) and an age-related decline in BV/TV (r =−0.46, p < 0.001, n = 53) as well asin the HV/TV (r = −0.318, p <0.05, n = 53). There was an age-related inversecorrelation between BV/TV and AV/TV (r =−0.58, p < 0.001). No significant correlation betweenthe AV/TV and the body mass index (r = 0.06, n.s.,n = 52) was detectable. Compared with age-matchedcontrols, patients with osteoporosis exhibited an increased AV/TV(P < 0.05) and decreased BV/TV (P < 0.05)but no statistically significant difference in HV/TV. Our datasupport the hypothesis that with aging and in osteoporosis an enhancedadipogenesis is observed in the bone marrow and that these changes areinversely correlated to decreased trabecular bone volume. The cellularand molecular mechanisms mediating these changes remain to bedetermined.